Several years ago it was reported by us that some patients on penicillin therapy, when transfused with donor blood containing hemagglutinating penicillin antibody, suffered an untoward reaction similar to anaphylaxis or serum sickness. We are attempting to establish a prospective, control study to determine if passively transferred antipenicillin antibody can induce allergic manifestations in patients who are receiving penicillin or penicillin derivatives as part of their therapy. Donors will be screened for a history of penicillin hypersensitivity and their blood will be tested for anti-penicillin antibodies by hemagglutination and radioimmunoassay techniques. Recipients of blood from donors with a history of penicillin hypersensitivity will be followed for untoward reactions following penicillin therapy and will also be tested for IgG and IgE anti-penicillin antibodies. Appropriate controls will be similarly followed.